Des changements dans la concentration de calcium altère les propriétés mécaniques du disque intervertébral

Introduction: Low back pain is often attributed to pathological intervertebral disc (IVD) mechanics. Since we recently discovered that increased free calcium (Ca2+) concentration in the human IVD leads to matrix degradation, we hypothesized that elevations in Ca2+ also impair IVD mechanics. The aim of this study was to determine the effect of free Ca2+ on the mechanical properties of the IVD.

Methods: Fresh caudal bovine IVDs were incubated for 4 weeks with either 1mM or 5mM Ca2+. Uncultured IVDs were used as control. All IVDs were subjected to compressive dynamic mechanical testing (frequencies ranging from 0.01Hz to 5Hz, 5% strain amplitude) using a multiaxial mechanical

tester (Mach-1 v500css, Biomomentum Inc, Canada) to assess IVDs bulk Young's modulus (E*). Subsequently, the IVDs were dissected in two halves by a sagittal cut and stiffness mapping of each half section was generated using an automated spherical indentation technique (R=3.2mm, 15 measures/cm2, amplitude=0.5mm).

Results: After a 4-week incubation in 1mM or 5mM Ca2+, E* decreased significantly compared to the control. The stiffness of the IVDs incubated with 1mM Ca2+ remained almost unchanged when compared to the control. However, at high Ca2+ concentration (5mM), the stiffness significantly (p<0.0001) decreased in annulus fibrous (AF) from 0.30±0.02MPa to 0.06±0.01MPa, while for both nucleus pulposus (NP) and endplates (EP), tissues stiffness significantly increased (p<0.0001) from 0.030±0.001MPa to 0.060±0.002MPa and from 1.45±0.15MPa to 2.5±0.07MPa, respectively.

Conclusions: High Ca2+ concentration alters IVD’s bulk and regional mechanical properties by decreasing the AF stiffness and increasing NP and EP stiffness. These results are consistent with our previous findings suggesting that increasing the calcium levels resulted in increases in the mineralisation of EP. Consequently, assessing mechanical properties of each IVD’s region may provide a better understanding of its bulk mechanical behavior and its integrity during degeneration.